Using Standard Curve to Estimate DNA Quantity - Forensic Focus #4

Learn that a standard curve is a tool to estimate DNA concentration in unknown samples by comparing them to standards with known concentrations. This method allows accurate quantification of DNA extracts.

Create a 10-fold dilution series of DNA standards ranging from 50 nanograms per microliter down to 5 picograms per microliter. During PCR cycles, measure the fluorescent signal for each standard and record the CT (cycle threshold) value when the signal crosses the detection threshold.

Plot the CT values for each dilution standard on a graph with log concentration on the x-axis and CT value on the y-axis. The software generates a regression line that fits the data points from the standard dilution series.

Assess the slope of the regression line, which measures PCR efficiency. An ideal slope of -3.3 indicates 100% PCR efficiency with target DNA doubling each cycle.

Review the y-intercept, which represents the expected CT value for a 1 nanogram per microliter sample. Evaluate the R-squared value, which measures how well the regression line fits the data points, with 1.0 being a perfect fit.

Prepare the standard dilution series with careful pipetting technique and run standards in duplicate to ensure consistent and accurate results. Proper preparation ensures the standard curve passes all quality metrics tests.

Measure the CT value of an unknown DNA sample and compare it to the validated standard curve to estimate the DNA concentration of the unknown sample. Once the standard curve passes quality metrics, it can reliably evaluate unknown samples.